Heretofore known attempts to provide a practical mechanized thinning or blocking method and device for sugar beets or other row crops have been unsuccessful for a number of reasons. The primary disadvantage or defect of prior art devices has been their inability to block on both sides of a single plant with any degree of accuracy. In effect, the devices have not been dependable in locating or isolating a plant and hence a grower could not be certain of obtaining a row with plants spaced uniformly from each other at a prescribed distance. The prior art machines left large gaps in a row and also left plants within an area that should have been a space. The unreliability and inconsistency of the machines has militated against their adoption and usage by row crop farmers. Another disadvantage arises in respect of machines that utilize a vacuum as part of a system for locating individual plants. That disadvantage is the picking up or suctioning of debris into the vacuum housing, including dirt, weeds, rocks and plants. Unless the vacuum system is precisely controlled and utilized the vacuum system rapidly becomes clogged or the fan or blower creating the vacuum becomes damaged because of the debris picked up by the vacuum.
In order to appreciate the significance of the desirability and need for accurate spacing of row crops, it is helpful to understand the problems of the grower. The inability of farmers to achieve equal spacing between plants has a direct effect on crop yield per acre. While adverse weather and plant disease are large problems for a grower, so too is the matter of prescribed spacing between plants. Agricultural authorities have established that for sugar beets, for instance, a plant spacing of 10 inches is best for maximum production. A field with erratic spacing, ranging from 4 to 20 inches, has the adverse result of reducing production by as much as 5 tons per acre. Using hand crews to thin row crops is almost prohibitively expensive, particularly where seeding is heavy which is more desirable from the grower's standpoint than light or thin seeding. Thus the grower is forced into the dilemma of resorting to expensive hand labor or to machines which have not been perfected and which lack precision in establishing accurate spacing as they thin out the excess plants.
Realistically, a farmer should plant seeds thickly or densely enough to assure a well populated row of plants and then thin and leave the desired 10 inches between each plant. It has been demonstrated that a vacuum style machine can be very accurate in thinning plants where populations of twenty plants per foot have germinated and grown to thinning size. This enables the machine to isolate and save a single plant and there is sufficient plant population in the row to assure that another one can be saved at the desired distance forward. Thus, it can be appreciated that the matter of thinning crops is profoundly important to the grower.
The only known prior art row crop thinning machine which is pertinent to the invention disclosed and claimed herein is U.S. Pat. No. 3,308,890 which issued Mar. 14, 1967. That patent shows a system in which the vacuum is applied constantly to the housing 16 by way of header 12 leading to the suction fan or blower. As noted above, the difficulty with the patented device is that it draws a large amount of debris into the vacuum system since there is no controlled utilization of the vacuum air. Other patents which are not pertinent to the invention but which are of interest are U.S. Pat. Nos. 4,117,888; 3,590,925; 3,533,474; 2,433,856; and 3,227,276.